Towards a mechanism for surface hydrophobization of paper - Effect of combinations of polyelectrolytes and polymer particles

Abstract: Paper materials are cost effective and light weighted, they can easily be recycled and their use as an alternative to plastics is advantageous from an environmental and sustainability perspective. However, competing with plastics for packaging applications is a challenge for cellulosic products. The material needs to be strong and stiff also when exposed to liquids or moisture during transportation and storage. To achieve this for paper materials, which are intrinsically hydrophilic due to the nature of the cellulose, they need to be hydrophobized. Packaging paper materials are often made from recycled fibers. The constitution of the paper matrix can therefore vary a lot and the addition of hydrophobic compounds to the pulp in the paper production process is difficult to optimize. Therefore the recent development in paper hydrophobization has been towards Surface modification, so-called surface sizing. There is a plethora of surface sizing Products and these products are very efficient in making the paper surface more water resistant, but there is a lack of fundamental knowledge on how they work. The aim of this licentiate project, which can be regarded as the first part of a doctoral thesis work, is to explore and identify which physicochemical properties of the formulation used for surface sizing are governing the efficiency. In surface sizing the particle suspension is first mixed with starch in solution. Starchparticles and starch is the subject of one study described in this thesis. In this study the interactions between starch and three types of particles, differing in the type of stabilizer used, are explored. The different stabilizers rendered the particles cationic, anionic or amphoteric. It was found that the cationic particles formed aggregates with the starch and that it is mainly the high molecular weight, highly branched amylopectin fraction of the starch that participates in the aggregation. The aggregate formation, as well as the relaxation kinetics, are also investigated and it was concluded that the amylopectin chains give rise to steric stabilization even at the mostdestabilized state, i.e. at maximum aggregation. The relaxation kinetics is found to be molecular weight dependent while the equilibrated state is not, leading to a proposed aggregation mechanism based on patchwise flocculation. Finally the efficiency in reducing the water uptake of test paper sheets is assessed. The cationic particles are the most efficient in decreasing the water uptake and the efficiency is enhanced by aggregation.